The present invention relates to coating compositions for pretreating metal surfaces. More particularly, the present invention is directed to aqueous coating compositions for providing durable, adhesive and corrosion-inhibiting coatings, as well as a method for pretreating metal substrates with such coating compositions.
Pretreatment of metal surfaces such as aluminum, ferrous and zinc surfaces with inorganic phosphate compositions and/or coatings by contacting such surfaces with an aqueous phosphating solution is well known. Such phosphate pretreatment processes protect the metal surface to a limited extent against corrosion, and serve as an effective base for the later application of organic coating compositions such as paint, lacquer, varnish, primer, synthetic resin, enamel, and the like.
Inorganic phosphate coatings generally are formed on a metal surface by means of an aqueous solution which contains phosphate ion and, optionally, certain auxiliary ions including metallic ions such as iron, sodium, manganese, zinc, cadmium, copper, lead, calcium-zinc, cobalt, nickel and antimony ions. These aqueous solutions also may contain non-metallic ions such as halide ions, nitrate ions, sulfate ions and borate ions. Recent advances in the pretreatment field have been directed to coatings derived from solutions containing a minimum of three metal cations such as zinc, cobalt, nickel, manganese, magnesium or calcium.
Chromium-free compositions have been proposed as rinse compositions for improving the quality of coated metal substrates. For example, U.S. Pat. No. 3,695,942 discloses a zirconium rinse for use with metal surfaces which have been pretreated with a phosphate conversion coating.
Aqueous pretreatment processes with coating compositions including an organic compound such as tannin, phosphate ions and an oxidizing agent are taught through U.S. Pat. No. 5,868,820. Such pretreatment processes and compositions typically require immersion or deposition of the coating composition at temperatures of 120xc2x0 F. Chromium-free compositions and titanium and zirconium compositions are disclosed as rinse compositions for application over the first coating composition. Such pretreatment processes and compositions involve multiple coatings in order to provide satisfactory results over a variety of different metal substrates.
U.S. Pat. No. 4,338,140 discloses coating compositions for improving corrosion resistance over metal surfaces such as aluminum cans, which includes dissolved hafnium and/or zirconium, fluoride, up to about 500 parts per million of a vegetable tannin compound, and optionally phosphate ions.
While prior art pretreatment processes can be effective, they typically require processing conditions involving elevated temperatures, and are typically useful for only selected metal substrates
Accordingly, there is a need for a pretreatment process which can be conducted at ambient conditions and which can provide effective properties for a variety of substrate materials.
The present invention includes an aqueous composition for pretreating metal substrates. The aqueous composition includes at least one hydroxy functional cyclic compound present in an amount of at least about 500 ppm, with the hydroxy functional cyclic compound being selected from the group consisting of hydroxy phenolic compounds and heterocyclic nitrogen-containing compounds having polyhydroxy functionality, such as a tannin, for example quebracho and/or mimosa tannins. The aqueous composition further includes phosphate ions, at least one oxidizer-accelerator, at least one Group IVB metal compound capable of converting to a metal oxide upon application to the metal substrate, and water. The Group IVB metal compound is preferably selected from the group consisting of hexafluorozirconic acid and hexafluorotitanic acid and their soluble salts.
The composition of the present invention may further include at least one disaccharide, such as those selected from the group consisting of lactose and sucrose.
The present invention further relates to a process for pretreating a metal substrate. The process includes contacting the substrate with an aqueous composition which includes at least one hydroxy functional cyclic compound present in an amount of at least about 500 ppm; phosphate ions; at least one oxidizer-accelerator; at least one Group IVB metal compound capable of converting to a metal oxide upon application to the metal substrate; and water. The process is particularly useful for improving corrosion resistance of a variety of substrates, including cold rolled steel, steel surfaces treated with any of zinc metal, zinc compounds and zinc alloys; aluminum; aluminum alloys; zinc-aluminum alloys; aluminum plated steel; and aluminum alloy plated steel. The process includes contacting the metal substrate with the aqueous composition, for example by immersion or by spray application. The process may further include a rinsing step, such as by rinsing the metal substrate with an aqueous solution containing silane or an epoxy derivative after contact with the aqueous composition.